Haemophilus influenzae type B, strain W-2, is highly resistant to ampicillin (MIC, 12.5 mug/ml). The ampicillin resistance of strain W-2 was transferred to an antibiotic-sensitive strain TF-2 (RifR, SmR) during mixed incubation on membrane filters at 36 C(transfeer frequency, 4.6 times 10(-5) per donor). Resistance was also transferred from the primary recipient to a secondary one (TF-3, EryR, NovR). The transfer frequency between these derivative strains was 10(-4) after incubation for 30 min. Resistance in strain W-2 remained even after growth in the presence of ethidium bromide or at an elevated temperature, although ampicillin resistance was lost from 13%-25% of transcipient cells after growth in broth. Strain W-2 and transcipients of ampicillin resistance had equivalent levels of beta-lactamase activity, while sensitive segregants and recipient strains demonstrated little or no enzyme activity. Transfer of ampicillin resistance between strains of H. influenzae is probably mediated by conjugation since transfer (1) requires cell-to-cell contact, (2) remains unchanged in the presence of DNase I, and (3) occurs in the absence of demonstrable bacteriophage.